1. Field of the Invention
The present invention relates to a differential limiting control apparatus for controlling differential limiting force applied between right and left wheels and between front and rear wheels of a four-wheel drive vehicle.
2. Description of the Related Art
As four-wheel drive vehicles, there have been known an on-demand four-wheel drive vehicle in which motive power from the engine is transmitted at all times to either the front wheels or the rear wheels and, when necessary, part of the motive power is transmitted also to the other of the front wheels and the rear wheels, and a full-time four-wheel drive vehicle in which the driving force is transmitted to the four wheels at all times. In some four-wheel drive vehicles, an electronic controlled differential limiting device is provided between the right and left wheels or between the front and rear wheels. The differential limiting device applies a suitable differential limiting force between the right and left wheels or between the front and rear wheels in accordance with the running condition of the vehicle and the rotational speed difference between the wheels, to make the motive power appropriately distributed among the wheels and thereby improve the turning performance and running stability of the vehicle.
In cases where the inner front wheel of an on-demand four-wheel drive vehicle, for example, slips due to excessive motive power transmitted thereto during turning, differential limiting control is carried out to restrain the inner wheel from slipping by producing a differential limiting force in accordance with the rotational speed difference between the right and left wheels.
Also, a technique for vehicles with no differential limiting device provided between the right and left wheels is known, wherein the differential limiting device arranged between the front and rear wheels is utilized to restrain slipping, as disclosed in Unexamined Japanese Patent Publication No. 2003-237398 (hereinafter referred to as Patent Document 1), for example. Where slipping of one of the four wheels is restrained by means of the differential limiting force applied between the front and rear wheels, the differential limiting force tends to be deficient because the rotational speed difference between the front and rear wheels is ½ that between the right and left wheels caused by the slipping. Taking this into account, in the technique disclosed in Patent Document 1, the differential limiting force is corrected to increase.
However, in situations where the inner front wheel is slipping, the average wheel speed of the right and left front wheels is higher than that of the right and left rear wheels because of the slipping of the inner wheel, and therefore, not only the differential limiting between the right and left wheels but also the differential limiting based on the rotational speed difference between the front and rear wheels are executed. During turning of a vehicle, a rotational speed difference occurs between the front and rear wheels due to an inner wheel turning track difference. Consequently, the differential limiting between the front and rear wheels hinders turning of the vehicle and furthers under-steering, giving rise to the problem that the turning performance of the vehicle lowers.
In the technique disclosed in Patent Document 1, the differential limiting force applied between the front and rear wheels is increased to restrain slipping. Increasing the differential limiting force, however, furthers under-steering and lowers turning performance, with the result that the problem remains unsolved.